Welcome to the Treehouse Community

The Treehouse Community is a meeting place for developers, designers, and programmers of all backgrounds and skill levels to get support. Collaborate here on code errors or bugs that you need feedback on, or asking for an extra set of eyes on your latest project. Join thousands of Treehouse students and alumni in the community today. (Note: Only Treehouse students can comment or ask questions, but non-students are welcome to browse our conversations.)

Looking to learn something new?

Treehouse offers a seven day free trial for new students. Get access to thousands of hours of content and a supportive community. Start your free trial today.

Java Java Data Structures Efficiency! Changing Course

fixedVideoTitle()

Here is the error:

./QuickFix.java:22: error: incompatible types: Object cannot be converted to Video newMap.put(newTitle, entry.getValue()); ^ Note: Some messages have been simplified; recompile with -Xdiags:verbose to get full output 1 error

I don't understand this because the value of the set is a video.

com/example/model/Course.java
package com.example.model;

import java.util.List;

public class Course {
  private String mName;
  private List<Video> mVideos; 

  public Course(String name, List<Video> videos) {
    mName = name;
    mVideos = videos;
  }

  public String getName() {
    return mName;
  }

  public List<Video> getVideos() {
    return mVideos;
  }

}
com/example/model/Video.java
package com.example.model;

public class Video {
  private String mTitle;

  public Video(String title) {
    mTitle = title;
  }

  public String getTitle() {
    return mTitle;
  }

  public void setTitle(String title) {
    mTitle = title;
  }

}
QuickFix.java
import com.example.model.Course;
import com.example.model.Video;
import java.util.List;
import java.util.HashMap;
import java.util.Map;

public class QuickFix {

  public void addForgottenVideo(Course course) {
    List<Video> newList = course.getVideos();
    // TODO(1):  Create a new video called "The Beginning Bits"
    Video video = new Video("The Beginning Bits");
    // TODO(2):  Add the newly created video to the course videos as the second video.
   newList.add(1, video);
  }

  public void fixVideoTitle(Course course, String oldTitle, String newTitle) {
   Map<String, Video> newMap = new HashMap<String, Video>(videosByTitle(course));
   for (Map.Entry entry : newMap.entrySet()) {
     if (entry.getKey() == oldTitle) {
       newMap.remove(entry.getKey(), entry.getValue());
       newMap.put(newTitle, entry.getValue());
     }
   }
  }

  public Map<String, Video> videosByTitle(Course course) {
    Map<String, Video> videoMap = new HashMap<String, Video>();
    for (Video video : course.getVideos()) {
      videoMap.put(video.getTitle(), video);
    }
    return videoMap;
  }

}

1 Answer

Umesh Ravji
Umesh Ravji
42,361 Points

Hi Kenneth, you can cast the entry.getValue() object to type Video but then you will encounter an ConcurrentModificationException.. read more about it here: http://stackoverflow.com/questions/12177889/java-util-concurrentmodificationexception-while-accessing-hashmap

public void fixVideoTitle(Course course, String oldTitle, String newTitle) {
  Video video = this.videosByTitle(course).get(oldTitle);
  // Update video title here. May want to check to see if video object isn't null first.
}

You just wrote a method videosByTitle(Course course) which returns a Map (mapping titles to videos), so use that method to find the video you need, rather than iterating over entire collection.

I don't know how to use videosByTitle without iterating over the collection. I don't know how to remove the oldvidoetitle and video and then put in the new video title and put the video back in without an error.

Umesh Ravji
Umesh Ravji
42,361 Points

I'll change the code a little to hopefully make it more clearer.

public void fixVideoTitle(Course course, String oldTitle, String newTitle) {
    Map<String, Video> videoMap = videosByTitle(course); // Step 1
    Video video = videoMap.get(oldTitle); // Step 2
    // Step 3
}
  1. Use the videosByTitle(Course course) to get the Map (that maps titles to video objects).
  2. Get the video object that needs to be updated using the get() method of the Map.
  3. Now that you have the video object, update it's title using the setTitle(String title) method. There is no need to remove or add any video objects, simply update the title property of the existing video object.